<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <!-- undefined  null Number String Boolean -->
    <script>
        //typeof 
        var a=undefined;
        console.log(typeof a);  //undefined
        var b=null;
        console.log(typeof b);  //object
        console.log(a==b);   //true
        console.log(a===b);  //false


        var num=100;
        var a=0.1;
        var b=0.2;
        console.log(a+b); //浮点数相加精度到小数点后17位
        console.log(typeof num);  //number


        var c=num.toString();
        console.log(typeof c);

        //string可以用单引或双引来
        console.log(c.length);//计算值得长度
        var d=3;
        var e=String(d);
        console.log(typeof e);

        //1+true=2   1+false=1     跟数字在一起计算时true=1   false=0
    </script>
</body>
</html>